Preparation of metal powder compacts prior to pressing



PREPARATION OF METAL POWDER COIVIPACTS PRIOR TO PRESSING 8 Claims. (Cl. 75-211) This invention relates to a method of preparing metal shapes by powder metallurgical techniques. More particularly it relates to a method of preparing metal shapes directly from compounds of the metal to be formed.

In accordance with the prior art practice in the production of uranium slugs metallic uranium, which has been previously cleaned with nitric acid, for example, is heated at a temperature of about 225 C. in an atmosphere of purified hydrogen until the entire metallic material has been converted to the hydride. The resultant hydride is in the form of an extremely fine powder occupying approximately five times the volume of the original powder material. This hydride is then decomposed in vacuum at about 275 C. for approximately 16 hours. The resultant substance is a sponge or sinter block of uranium powder. This sinter block must then be ground to break up the agglomerates of extremely fine particles of uranium. The comminuted powder which is usually further ground in a mortar and pestle is then ready for pressing which is normally carried out at pressures ranging from 30 to 100 t. s. i. The pressed specimens can then be sintered in vacuum for three hours at about 1100 C. to obtain specimens which vary in density from about 85 to 90% theoretical. The most dense of these can then be repressed to increase the density to a maximum of about 94.5%.

This process presents several difficulties in that the die damage at the high repressing pressures is so extensive as to limit the practical applications of this method. Furthermore, it has been shown that the comminution of the sinter block obtained as a result of the reduction of the hydride has a marked effect on the pressed densities of the specimen made from the powder.

It is, therefore, an object of this invention to improve the technique of preparing metallic slugs from the compounds of metals in order to obtain slugs of maximum density.

It is a further object of this invention to simplify the techniques of preparing metallic slugs from the metal compound While producing slugs of maximum density.

It is a further object of this invention to develop a method of preparing sl'ugs of metal approaching theoretical density from the hydride of the metals without resorting to high repressing pressures.

It has been found that these objects and other advantages incidental thereto can be obtained by reducing the compounds in a vessel shaped to coincide with a die, transferring the reduced preform so obtained into a die and then subsequently pressing and sintering the reduced mass.

The method of this invention can be carried out in several ways. For example, the compound can be placed in a boat which is shaped to conform to the die cavity in which the metal is subsequently to be sintered and pressed. The boat can then be placed in an evacuated chamber wherein the decomposition is carried out at the normal temperatures for such operations. In the case of uranium hydrides, for example, the temperature would powder.

lie in the range of approximately 275 C. The preforms so obtained are quite fragile and are somewhat difficult to handle. However, once inserted in the die they show the advantage of not having to be leveled. This operation is normally a tedious one when working with The preforms of course can be pressed without further grinding thus eliminating the effects which the different modes of grinding may have on the final product. In order to eliminate handling-however, it is possible to place the uranium powder directly in the die body in which it is later to be pressed and sintered. When this is done the material can be treated in the same manner as if preforms were to be'made except that the handling between the formation of the preforms and the insertion into the die body is avoided.

Preforms which were formed. in this manner and pressed at pressures at 60 t. s. i., sintered and vacuum treated for three hours at 190 C. exhibited densities after pressing of about 89.2% and after sintering. the density had increased to approximately 93%. When repressed at pressures of only 100 t. s. i., they exhibited densities above 95% of the theoretical. In the case of uranium, for example, metallographic examinations revealed at least two differences between conventional and preforming methods of producing uranium bars. The inclusions in the conventionally prepared bar had been torn out during metallographic preparation whereas they appear to be held in the specimen prepared by preforming. Secondly, in the bars prepared in accordance with prior art techniques fine gray'lines are visible whereas none are apparent in those bars prepared. from the hydrides. These gray lines appear to be barriers to the self-diffusion of the metal and might well account for the lower densities which are obtained by the prior art methods. It is, of course, to be understood that this method of preparing a metallic slug from the hydride is not limited to the preparation of uranium slugs but might readily be used with such metals as thorium, protactinium, beryllium and zirconium. I

In its essential steps the method of this invention differs only from the prior art in that it eliminates the grinding of the sintered hydride. This is accomplished by carrying out the decomposition of the hydride in a preshaped body, pressing the preform thus produced and then subsequently sintering it to form the finished slug of metal. The temperatures and times of reaction can be quite similar to those used in prior art techniques and depend upon the material which is being handled. The densities of the final specimens are, however, materially increased over those obtained at similar temperatures and pressures with prior art techniques.

Since the metal prepared in this manner has not been comminuted after reduction of the hydride, the danger of contaminating the metal with oxygen or other elements of which the comminuter is made is eliminated. Furthermore, this method also eliminates the tiresome necessity of levelling the powdered metal in the die body prior to pressing and sintering.

Shaped uranium pieces are a source of alpha particles or, with an auxiliary source of thermal neutrons and a neutron moderator, they are a means of multiplying neutrons. Shaped zirconium pieces are useful in electronics (Espe et al., Electronics, 23, (1950)), and in surgical applications (Bates et al., Surg., Gynecol. and Obstet, 87, 212 (1948)). Shaped beryllium pieces are useful in moderating neutrons (Patent Number 2,206,634, Fermi et al.).

While the above description submitted herewith discloses a preferred and practical embodiment of the method of this invention in making metallic shapes it will be understood that the specific details described are by way of illustration and are not to be construed as limiting the scope of the invention.

What is claimed is:

1. The method of fabricating uranium by powder metallurgical techniques which comprises the sequence of steps of introducing uranium hydride in powder form into a receptacle shaped to coincide with the contour of the die cavity, heat treating the hydride below the melting point of uranium under conditions that will decompose the hydride to metal and cause the metal to cohere in the shape of the receptacle and thereafter pressing the preformed metal powder and sintering it in vacuum to obtain a dense sintered compact.

2. The method of fabricating zirconium by powder metallurgical techniques which comprises the sequence ofisteps of introducing zirconium hydride in powder form into a receptacle shaped to coincide with the contour of the die cavity, heat treating the hydride below the melting point of zirconium under conditions that will decompose the hydride to metal and cause the metal to cohere in the shape of the receptacle and thereafter pressing the preformed metal powder and sintering it in vacuum to obtain a dense sintered compact.

'3.i The method of fabricating beryllium by powder metallurgical techniques which comprises the sequence of steps of introducing beryllium hydride in powder form into a receptacle shaped to coincide with the contour of the die cavity, heat treating the hydride below the melting point of beryllium under conditions that will decompose the hydride to metal and cause the metal to cohere in the shape of the receptacle and thereafter pressing the preformed metal powder and sintering it in vacuum to obtain a dense sintered compact.

4. The method of fabricating uranium by powder metallurgical techniques which comprises the sequence of steps of introducing uranium hydride in powder form into a receptacle shaped to coincide with the contour of the die cavity, heat treating the hydride at 275 C. until the hydride is reduced to the metal and thereafter pressing the preformed uranium metal powder and 4 into a receptacle shaped to coincide with the contour of the die cavity, heat treating the hydride at 275 C. until the hydride is reduced to the metal and thereafter pressing the preformed uranium metal powder at about t. s. i. and sintering it in vacuum to obtain a dense sintered compact.

6. The method of fabricating uranium by powder metallurgical techniques which comprises the sequence of steps of introducing uranium hydride in powder form into a receptacle shaped to coincide with the contour of the die cavity, heat treating the hydride at 275 C. until the hydride is reduced to the metal and thereafter pressing the preformed uranium metal powder at about 60 t. s. i. and sintering it in vacuum at a temperature of about 1090 C. to obtain a dense sintered compact.

7. The method ,of fabricating uranium by powder metallurgical techniques which comprises the sequence of steps of introducing uranium hydride in powder form into a receptacle shaped to coincide with the contour of the die cavity, heat treating the hydride at 275 C. until the hydride is reduced to the metal and thereafter pressing the preformed uranium metal powder at about 60 t. s. i. and sintering it in vacuum at a temperature of about 1090 C. to obtain a dense sintered compact and then repressing the sintered compact at a pressure of about t. s. i- I 8. The method of fabricating metal by powder metallurgical techniques which comprises the sequence of steps of introducing the powdered hydride of said metal into a receptacle shaped to coincide with the contour of a die cavity, heat treating the hydride at a temperature below the melting point of said metal under conditions that will decompose the hydride to metal powder and cause the metal powder to cohere in the shape of the receptacle and finally pressing the preformed metal powder in the die cavity and then sintering it to obtain a dense sintered compact.

References Cited in the file of this patent UNITED STATES PATENTS 2,254,549 Small Sept. 2, 1941 2,287,951 Tormyn June 30, 1942 2,446,780 Newton Aug. 10, 1948 2,588,153 Newton Mar. 4, 1952 

1. THE METHOD OF FABRICATING URANIUM BY POWDER METALLURGICAL TECHNIQUES WHICH COMPRISES THE SEQUENCE OF STEPS OF INTRODUCING URANIUM HYDRIDE IN POWDER FORM INTO A RECEPTACLE SHAPED TO COINCIDE WITH THE CONTOUR OF THE DIE CAVITY, HEAT TREATING THE HYDRIDE BELOW THE MELTING POINT OF URANIUM UNDER CONDIDTIONS THAT WILL DECOMPOSE THE HYDRIDE TO METAL AND CAUSE THE METAL TO COHERE IN THE SHAPE OF THE RECEPTACLE AND THEREAFTER PRESSING THE PREFORMED METAL POWDER AND SINTERING IT IN VACUUM TO OBTAIN A DENSE SISNTERED COMPACT. 